Facts about absorbing souls:
USER = the one who absorbed the soul
OWNER = the spirit the soul is connected to.
When a soul is absorbed, it is stored in the body alongside the main soul (standardly, this is represented as it being below the main soul physically)
The absorbed soul is used as a sort of "battery", allowing the user to pull from that soul for magic and energy. this is an exhausting process for the absorbed, and if strained too much, can crack or even break the soul, since it only has a loose structural connection to the users body, the soul itself is still extremely fragile.
An absorbed soul's owner is not allowed to move on to the afterlife until their soul breaks as the spirit of the soul's owner is trapped inside of the soul itself.
Absorbing a soul does not transform the user in most cases. (barring exceptions like Arirong)
Absorbing souls is no longer dependant on species, although, absorbing a soul the same type as your own has consequences (much like cannibalization)
It was a common myth that absorbing souls would bring the user closer to GODHOOD, although that may have been true at one point, the ROARING's altering of reality seems to have changed that if it was the case.
It is not possible for an absorbed soul's owner to communicate with it's user. if the user allows, even subconsciously, it is however possible for a soul's owner to influence it's user in many ways, in cases like Arirong's, even deliberate control of their limbs, like a sort of impulse reaction from the absorbed soul itself.
(Apologies for the formatting of this post compared to the others! I meant to have visuals alongside this post, but when you can't manage to get it done, you gotta compromise! Enjoy the lore post!)